The following description will explain a conventional image forming apparatus of an electrophotographic method. FIG. 14 is a schematic view showing the configuration of a conventional image forming apparatus.
As shown in FIG. 14, the image forming apparatus includes a charging section 91, a developing section 92, a transfer section 93, a cleaning section 94, and a discharging section 95 around a photosensitive drum 90 that is an image information forming body composed of an aluminum base body, etc. whose surface is coated with an organic photoconductive layer (hereinafter, will be referred to as an OPC layer), Se, a-Si, etc.
In the image forming apparatus, as the photosensitive drum 90 rotates in the direction A as shown in FIG. 14, the surface of the photosensitive drum 90 is uniformly charged by corona discharge, etc. of the charging section 91 and then exposed by an exposure section (not shown) composed of an image scanner, an LED, etc. in accordance with image information, thereby forming electronic latent images such as an electrostatic latent image, an electric charge latent image, and a conductive latent image thereon.
Toner 81 including one or two components is supplied from the developing section 92 to the electronic latent image, which is thus visualized and forms a toner image. The toner 81 is charged fine particles colored with carbon black, a pigment, etc., averaging 7 .mu.m to 15 .mu.m in particle diameter, and containing a polyester resin, a polypropylene resin, a styrene-acrylic copolymer, etc. as binder resins.
As a transfer sheet 96 that is a transfer material is transported between the photosensitive drum 90 and the transfer section 93 from the right side of FIG. 14 by a paper supply section (not shown), the toner 81 that has formed a visual image on the surface of the photosensitive drum 90 is transferred from the photosensitive drum 90 onto the transfer sheet 96 by corona discharge of the transfer section 93.
The transfer sheet 96 onto which the toner 81 has been transferred is ejected on the left side of FIG. 14 by a paper ejecting section (not shown). The toner 81 is heated and pressed by a fixing section (not shown) to melt, and the image that has been formed with toner on the transfer sheet 96 is fixed.
On the surface of the photosensitive drum 90 after the toner 81 is transferred onto the transfer sheet 96, there remains toner that has not been transferred. This is because not the whole toner image formed on the surface of the photosensitive drum 90 is transferred in the transfer stage where the transfer section 93 transfers the toner 81 onto the transfer sheet 96 in the image forming processes. Normally the toner 81 is transferred onto the transfer sheet 96 at about 80% efficiency with the remaining about 20% being left on the surface of the photosensitive drum 90 as residual toner 82 in the transfer stage. The residual toner 82 is cleaned off the surface of the photosensitive drum 90 by a cleaning section 94.
A cleaning blade made of an elastic member such as urethane rubber, or a fur brush composed of a brush with bristles made of a high polymer organic material, etc. such as nylon and acrylic is used as the cleaning section 94. Cleaning is executed by, for example, the tip of the cleaning blade or an elastic roller that scrapes the residual toner 82 and other adhering substances 83 off the surface of the photosensitive drum 90 when pressed against, or brought into contact with, the surface of the photosensitive drum 90.
The discharging section 95, typically disposed downstream from the cleaning section 94, induces discharging of the surface of the photosensitive drum 90 with light or corona discharge and thus eliminates unnecessary electric charge therefrom.
The image forming apparatus has the above explained configuration. Next, a transfer stage by the transfer section 93 will be explained.
A corona transfer method has been typically employed conventionally with the transfer section 93. According to that method, a transfer electric field is formed by charging the transfer sheet 96 from the backside of the transfer sheet 96 oppositely to the polarity of the toner 81 with corona discharge using a corotron charger, and then the toner 81 is transferred onto the transfer sheet 96 by the Coulomb's force.
However, in recent years, there is a greater interest in other methods, such as a roller transfer method and a belt transfer method, than in the corona transfer method using a corotron charger. According to the roller transfer method, the transfer is carried out by the Coulomb's force, as an elastic roller called a transfer roller provided on the surface thereon with a conductor or dielectric is pressed to the photosensitive drum 90 on the backside of the transfer sheet 96, and then a transfer electric field is formed by applying a bias voltage to the elastic roller (see for instance Japanese Laid-Open Patent Application No. 50-32947/1975 (Tokukaisho 50-32947) and Japanese Laid-Open Patent Application No. 56-110967/1981 (Tokukaisho 56-110967)). According to the belt transfer method, the transfer is carried out by the Coulomb's force as a transfer electric field is formed by charging an endless belt called a transfer belt (see for instance Japanese Laid-Open Patent Application No. 63-83762/1988 (Tokukaisho 63-83762), Japanese Laid-Open Patent Application No. 1-113771/1989 (Tokukaihei 1-113771), and Japanese Laid-Open Patent Application No. 2-46474/1990 (Tokukaihei 2-46474)).
The roller transfer method and the belt transfer method have advantages of creating less ozone than the conventional corona transfer method and of eliminating the need for a discharging section indispensable in the corona transfer method. Especially, as for the belt transfer method, the transfer sheet 96 is attracted toward the transfer belt due to dielectric polarization and preliminary charging and therefore transported while firmly adhering to the transfer belt and contacting with the photosensitive drum 90. The toner image is transferred in that state. Therefore, the transfer belt doubles as a transport section, and the transfer belt is more easily separated from the surface of the photosensitive drum 90 after the transfer is finished than the transfer section of the corona transfer method. Besides, although having a complex structure, the belt transfer method is often used for color image forming apparatuses, etc. because of its high freedom in setting a transfer area.
However, the belt transfer method using the transfer belt has problems: for example, a varying resistance value of the transfer belt depending on the environments, residual electric charges caused by non-uniform properties due to a problem in molding and processing of the belt, and a dirty backside of the transfer sheet 96 due to the residual electric charges (see Japanese Laid-Open Patent Application No. 2-179670/1990 (Tokukaihei 2-179670), and Japanese Laid-Open Patent Application No. 5-113725/1993 (Tokukaihei 5-113725)). Also, since electric charges having the opposite polarity to the toner are separated all of a sudden for example when the transfer sheet 96 enters into the photosensitive drum firm adhesion section and when the transfer sheet 96 is separated from the photosensitive drum 90 after the transfer, abnormal discharge (atmospheric discharge such as detaching discharge) at that time suddenly changes the attraction of the toner to the paper, the toner image on the transfer sheet 96 becomes unstable, and the toner is likely to be projected, resulting in poor quality in the finished image. The toner projection occurs also when the transfer sheet 96 is separated from the transfer belt.
The toner projection may be possibly reduced by the use of a conductive transfer belt (hereinafter, will be referred to as a conductive belt). However, the conductive belt has a low surface resistance and exhibits large electric charge leak in the surface plane of the transfer belt in a very humid and hot environment, adversely affecting transfer properties.
Also, as shown in FIG. 15, if there exists a pin-hole 90b in an OPC layer 90a of a photosensitive drum 90 facing a transfer belt 100 while a bias is being applied to the transfer belt 100, the electric charge in quite a large area around the pin-hole 90b flows from the transfer belt 100 to the aluminum base body 90c of the photosensitive drum 90 through the pin-hole 90b. The transfer belt 100 is therefore in a quasi-grounded state. The area around the pin-hole 90b is discharged, failing to form a transfer electric field and to transfer the toner 81 onto a transfer material. Especially, when using a transfer belt 100 having low surface and volume resistivities, a very large area extending in a direction perpendicular to the paper transporting direction (in a direction perpendicular to the transport direction of the transfer belt 100, that is, in the same direction as the direction of the shaft of the photosensitive drum 90) turns into the above-mentioned quasi-grounded state on the instant when the transfer is to be carried out to the place where the pin-hole 90b occurs. Resultant problems include failure in transfer due to weakening of the transfer electric field across that area, and electric current flowing in excess into the aluminum base body 90c of the photosensitive drum 90 through the pin-hole 90b.
In addition, the transfer sheet is well attracted if a dielectric belt with a high resistivity is used. However, if a conductive belt is used, the belt is charged for an extremely short period of time, and accordingly the transfer sheet is attracted for a shorter period of time. Therefore, there occurs a problem in paper transportation.